The present invention generally pertains to electroluminescent display devices and is particularly directed to improving the drive circuit of an LED display device so as to provide an apparent uniform brightness as to characters that are displayed in different digit sections of the display device.
A typical LED display device includes a plurality of digit sections. Each of the digit sections includes a plurality of LED segments in an array for providing illuminated displays of selected alphanumeric characters. Referring to FIG. 1, each digit section typically includes at least seven segments "a" through "g" for displaying numerals "0" through "9".
FIG. 2 shows a portion of a typical drive circuit 10 for one digit section. Each of the LED segments "a" through "g" is connected in series with a P-type MOSFET semiconductor switch 11 through 17. When the switch 11 is gated closed, the anode of the LED segment "a" is coupled to a source of voltage potential V.sub.DD. The anodes of the other LED segments "b" through "g" are coupled to the source of voltage potential V.sub.DD by the switches 12 through 17 in a like manner.
The cathodes of all of the LED segments "a" through "g" are connected in common and are coupled to a source of voltage potential V.sub.SS by an N-type MOSFET semiconductor switch 19.
A digit section is enabled for display when the switch 19 is gated closed in response to a predetermined first input signal. Typically the first input signals are multiplexed signals for sequentially enabling the different digit sections.
Given LED segments in a given digit secton are enabled for displaying a selected character when their respective series connected semiconductor switches 11 through 17 are gated closed in response to a predetermined combination of second input signals. Typically the second input signals are provided by a character generating circuit.
Accordingly, given LED segments in a given digit section are enabled to display a selected character in response to predetermined combinations of first and second input signals.
With such a typical drive circuit there is an apparent difference in the brightness of the different characters being displayed in the different digit section of the LED display when one character is made up of considerably more illuminated segments than another. To understand this apparent difference in brightness, reference is made to FIG. 3, which is an equivalent circuit or a portion of a drive circuit for one digit section. Each MOSFET semiconductor switch is a poor current source and is considered as equivalent to a perfect switch in series with a series resistance. R.sub.P11 through R.sub.P17 represent the internal resistances of the P-type MOSFET's 11 - 17. R.sub.N represents the internal resistance of the N-type MOSFET 19. The potential difference between V.sub.DD and V.sub.SS typically is provided by a battery, and is represented in the equivalent circuit by a DC voltage source V in series with a resistance R.sub.B. R.sub.B represents the internal resistance of the battery, and V.sub.B represents the battery voltage.
When a greater number of LED segments in a digit section is enabled for displaying a selected character, the current through the resistances R.sub.N and R.sub.B is increased. This results in a greater voltage drop across the series combination of R.sub.N and R.sub.B. Therefore, there is a smaller voltage drop across the resistances R.sub.p of whichever ones of the switches 11 through 17 that have been gated closed, and less current flow through whichever LED segments that are illuminated. Since the brightness of an LED segment is greater with greater current flow through the LED segment, the result is that digit sections having a significantly larger number of LED segments enabled for display appear dimmer than do digit sections having only a few segments enabled.
Another factor which affects the level of current flow through the LED segments is the internal resistance R.sub.B of the battery. When the number of illuminated LED segments is greater, the increased current flow through the battery internal resistance R.sub.B causes the voltage drop across the battery internal resistance R.sub.B to increase. This in turn decreases the applied potential difference V.sub.DD - V.sub.SS, and thereby provides a lower voltage for driving the MOSFET semiconductor switches 11-17 and 19. The effect of a lower voltage for driving the MOSFET's is that the equivalent resistances of the MOSFET's increase; and as a result the greater the number of segments that are illuminated, the greater is the resistance to current flow that is provided by the N-type MOSFET switch 19 and by those P-type MOSFET switches that are in series with an illuminated LED. Thus the effect of the battery internal resistance is to further increase the resistance to current flow in the illuminated LED segments as the number of illuminated LED segments is increased.